Octahedrite

{{see also|Widmanstätten pattern#Lamellæ formation mechanism|Widmanstätten pattern#Shape and orientation}}

Octahedrites derive their name from the crystal structure paralleling an octahedron. Opposite faces are parallel so, although an octahedron has 8 faces, there are only 4 sets of kamacite plates.

Due to a long cooling time in the interior of the parent asteroids, these alloys have crystallized into intermixed millimeter-sized bands (from about 0.2 mm to 5 cm).{{cite journal |title=Iron meteorites: Crystallization, thermal history, parent bodies, and origin |journal=Chemie der Erde – Geochemistry |volume=69 |issue=4 |pages=293–325 |doi=10.1016/j.chemer.2009.01.002 |year=2009 |last1=Goldstein|first1=J.I |last2=Scott|first2=E.R.D |last3=Chabot|first3=N.L |bibcode=2009ChEG...69..293G }} When polished and acid etched the classic Widmanstätten patterns of intersecting lines of lamellar kamacite, are visible.

In gaps between the kamacite and taenite lamellae, a fine-grained mixture called plessite is often found. An iron nickel phosphide, schreibersite, is present in most nickel-iron meteorites, as well as an iron-nickel-cobalt carbide, cohenite. Graphite and troilite occur in rounded nodules up to several cm in size.Vagn F. Buchwald: Handbook of Iron Meteorites. University of California Press, 1975.

File:ZacatecasMeteoriteGDL cropped.JPG

{{see also|Iron meteorite#Structural classification}}

Octahedrites can be grouped by the dimensions of kamacite lamellae in the Widmanstätten pattern, which are related to the nickel content:James H. Shirley,Rhodes Whitmore Fairbridge, Encyclopedia of planetary sciences, Springer, 1997. {{ISBN|978-0-412-06951-2}}

• Coarsest octahedrites, lamellae width >3.3 mm, 5–9% Ni, symbol Ogg
• Coarse octahedrites, lamellae 1.3–3.3 mm, 6.5–8.5% Ni, symbol Og
• Medium octahedrites, lamellae 0.5–1.3 mm, 7–13% Ni, symbol Om
• Fine octahedrites, lamellae 0.2–0.5 mm, 7.5–13% Ni, symbol Of
• Finest octahedrites, lamellae <0.2 mm, 17–18% Ni, symbol Off
• Plessitic octahedrites, kamacite spindles, a transitional structure between octahedrites and ataxites,Geochimica et Cosmochimica Acta, Volume 45, Ed. 9–12 9–18% Ni, symbol Opl

Octahedrite is an obsolete synonym for anatase, one of the three known titanium dioxide minerals.{{cn|date=September 2023}}

• Glossary of meteoritics

{{Reflist}}

• [http://webmineral.com/data/Kamacite.shtml Webmineral]
• [http://www.amonline.net.au/geoscience/meteors/types.htm Meteorites Australia]

{{Meteorites}}